The companies complied with a so-called "second request" for information from the Federal Trade Commission, Medco said in a securities filing. Such a second request for information is frequently made when potential competitive issues exist on a deal.

Express Scripts is seeking to acquire rival Medco to become easily the biggest U.S. company in managing prescription benefits, but the deal has drawn significant antitrust scrutiny. The companies still expect the deal will be completed in the first half of the year, Medco said.